Microfluidics deals with precise control and manipulation of fluids that are geometrically constrained to small, typically microliter, volumes. Because of the rapid kinetics and the potential for automation, microfluidics can potentially transform routine bioassays into rapid and reliable tests for use outside of the laboratory. Recently, a new paradigm for miniaturized bioassays has been emerged called “digital” (or droplet based) microfluidics. Digital microfluidics (DMF) relies on manipulating discrete droplet of fluids across a surface of patterned electrodes.1-10 This technique is analogous to sample processing in test tubes, and is well suited for array-based bioassays in which one can perform various biochemical reactions by merging and mixing those droplets. More importantly, the array based geometry of DMF seems to be a natural fit for large, parallel scaled, multiplexed analyses. In fact, the power of this new technique has been demonstrated in a wide variety of applications including cell-based assays, enzyme assays, protein profiling, and the polymerase chain reaction.
Unfortunately, there are two critical limitations on the scope of applications compatible with DMF—biofouling and interfacing. The former limitation, biofouling, is a pernicious one in all micro-scale analyses—a negative side-effect of high surface area to volume ratios is the increased rate of adsorption of analytes from solution onto solid surfaces. We and others have developed strategies to limit the extent of biofouling in digital microfluidics, but the problem persists as a roadblock, preventing wide adoption of the technique.
The second limitation for DMF (and for all microfluidic systems) is the “world-to-chip” interface—it is notoriously difficult to deliver reagents and samples to such systems without compromising the oft-hyped advantages of rapid analyses and reduced reagent consumption. A solution to this problem for microchannel-based methods is the use of pre-loaded reagents. Such methods typically comprise two steps: (1) reagents are stored in microchannels (or in replaceable cartridges), and (2) at a later time, the reagents are rapidly accessed to carry out the desired assay/experiment. Two strategies have emerged for microchannel systems—in the first, reagents are stored as solutions in droplets isolated from each other by plugs of air11 or an immiscible fluid12,13 until use. In a second, reagents are stored in solid phase in channels, and are then reconstituted in solution when the assay is performed.14-16 Pre-loaded reagents in microfluidic devices is a strategy that will be useful for a wide range of applications. Until now, however, there has been no analogous technique for digital microfluidics.
In response to the twin challenges of non-specific adsorption and world-to-chip interfacing in digital microfluidics, we have developed a new strategy relying on removable polymer coverings.17-19 After each experiment, a thin film is replaced, but the central infrastructure of the device is reused. This effectively prevents cross-contamination between repeated analyses, and perhaps more importantly, serves as a useful medium for reagent introduction onto DMF devices. To demonstrate this principle, we pre-loaded dried spots of enzymes to the plastic coverings for subsequent use in proteolytic digestion assays. The loaded reagents were found to be active after >1 month of storage in a freezer. As the first technology of its kind, we propose that this innovation may represent an important step forward for digital microfluidics, making it an attractive fluid-handling platform for a wide range of applications.